This invention relates to an improvement in telephone line testing equipment of the type described in U.S. Pat. No. 3,843,848. Other improvements to this patent were applied for in two pending patent applications titled "Telephone Line Testing Equipment" filed on Sept. 14, 1973 by Russell G. Cox et al., Ser. No. 397,268 now abandoned and "Coded Telephone Line Testing Equipment" filed on Jan. 14, 1974 by Brian B. Brady, Ser. No. 432,806. The subject matter of these applications and the patent relates to a highly effective method of remotely testing telephone lines by passing alternating current signals down the line from a central office and detecting the alternating current at a remote point in the circuit with a suitable pickup. The sensed AC signal is used to operate a switch so as to connect the telephone lines together to form a loopback circuit through which the lines up to that remote point may be tested. The subject matter of these patent applications and the patent are herein incorporated by reference. Although these prior inventions work well there exist additional requirements which can be more suitably satisfied by the invention proposed herein.
The last mentioned patent application relating to coded telephone line testing equipment teaches a system wherein a series of coded tone bursts are transmitted down the line and decoded at the remote end to determine if the particular remote unit in question is the one being addressed. If a match is made, a loopback circuit is completed for that piece of equipment only and the rest of the circuit is not affected. This is very useful in four-wire telephone systems wherein it is common to connect a plurality of different sets of equipment to a single circuit. With the use of this prior invention it is possible to loopback only one particular piece of equipment. In addition, it provides a greater degree of security from the noisy environment normally associated with computers and other modems that may be connected to the telephone line. However, the subject matter of the aforementioned patent application turns out to be somewhat more expensive and more complex than the present invention because it uses a clock signal generated at the transmitting station which is transmitted down the line to the remote unit. The clock signal is used by the remote unit to help compare the incoming code signal with the stored code to determine whether or not the loopback signal is intended for that particular remote unit. The present invention avoids this extra complication as described hereinafter.